1. Field of the Invention
The invention relates in general to the field of construction, and specifically to improved apparatus and methods for seismic retrofitting concrete structures.
2. Description of the Related Art
Retrofitting of existing concrete structures is often necessary to meet improved building safety codes. For example, in regions of the world susceptible to earthquakes, building codes are continually examined and modified by the appropriate regulatory agencies to require improved structural resilience to seismic activity by retrofitting the existing structure to provide additional stability and resilience to seismic vibrations.
Seismic retrofitting of an existing concrete structure is often a large undertaking with significant inconveniences to the occupants of the concrete structure. Some retrofitting procedures comprise strengthening the concrete structure by coupling additional concrete and/or steel (to provide ductility). Other retrofitting procedures comprise isolating the concrete structure from the ground by installing shock absorbing systems. Typically, such construction projects entail high levels of noise, dust, pollution, vibration, and general disruption to the normal operations of the concrete structure. These inconveniences are especially troublesome for structures such as hospitals, where the occupants are especially sensitive to any disruptions, and relocation for the duration of the construction project is generally not feasible.
Mechanical drilling of concrete is an especially disruptive component of the retrofitting of concrete structures. Typically, such mechanical drilling is accomplished by using diamond-tipped rotary drills or impact drills, which drill by brute physical contact with the concrete surface. These types of mechanical drills produce high levels of noise, significant vibrations which propagate to other parts of the structure, and substantial amounts of dust and debris which require special protective measures.
Lasers have been used in exotic construction projects, because of their ability to cut a wide variety of materials and their applicability to hazardous or extreme conditions. For example, in U.S. Pat. No. 4,227,582 (“the '582 patent”) issued to Price and incorporated in its entirety by reference herein, Price discloses an apparatus and method for perforating a well casing and its surrounding formations from within the confined area of an oil or gas well. In the '582 patent, the laser drilling tool is used in conjunction with a high pressure injection of exothermic gases (e.g., oxygen) and fluxing agents (e.g., powdered iron or alkali halides) which react with the drilled material to speed up the drilling process. In addition, U.S. Pat. No. 4,568,814 (“the '814 patent”) issued to Hamasaki et al., and incorporated in its entirety by reference herein, discloses an apparatus and method for cutting concrete in highly hazardous contexts, such as for the dismantling of a biological shield wall in a nuclear reactor. The '814 patent also discloses the use of an automated laser cutter in the conjunction with MgO-rich supplementary materials and a cleaning device to facilitate the removal of the viscous molten slag produced by the cutting process.
A study of the cutting ability of a carbon dioxide laser as a function of numerous parameters to cut concrete and reinforced concrete has been performed by Yoshizawa, et al. entitled “Study on Laser Cutting of Concrete” and published in the April 1989 “Transactions of the Japan Welding Society,” Vol. 20, No. 1, p. 31 (hereafter referred to as “the Yoshizawa article”), which is incorporated in its entirety by reference herein. The Yoshizawa article provides data from laboratory experiments which monitored the depth of cuts generated by the laser as a function of laser power, assist gas pressure and direction, laser lens focal length, scanning speed of the laser spot across the concrete, and types and water content of the concrete. In addition, the Yoshizawa article concluded that laser energy densities greater than approximately 106 W/cm2 are necessary to cut concrete, and laser energy densities greater than approximately 107 W/cm2 are necessary to cut steel-reinforced concrete.